1. Field of the Invention
The present invention relates to a communication system using an Orthogonal Frequency Division Multiple Access (OFDMA) scheme, and more particularly to an apparatus and a method for controlling transmit power of a preamble sequence for an Adaptive Antenna System (AAS).
2. Description of the Related Art
In a 4th generation (4G) communication system, which is the next generation communication system, research is being performed to provide users with services having various Qualities of Services (QoSs) at a high transmission speed. In particular, in the current 4G communication system, research is being performed to support a high speed service for ensuring mobility and QoS in a Broadband Wireless Access (BWA) communication system such as a wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system. Representative communication systems of the 4G communication system are the Institute of Electrical and Electronics Engineers (IEEE) 802.16d communication system and the IEEE 802.16e communication system.
The IEEE 802.16d communication system and the IEEE 802.16e communication system utilize an Orthogonal Frequency Division Multiplexing (OFDM) scheme/an OFDMA scheme to support a broadband transmission network for a physical channel of the wireless MAN system. The IEEE 802.16d communication system considers only a single cell structure and stationary subscriber stations (SSs), which means the system does not accommodate the mobility of the SSs at all. However, the IEEE 802.16e communication system accommodates the mobility of an SS in the IEEE 802.16d communication system.
The IEEE 802.16e communication system expands a cell service area by means of a multi-antenna, and uses a Space Division Multiple Access (SDMA) scheme for increasing the total capacity. In order to use the SDMA scheme, it is necessary to design a preamble sequence so that Channel Quality Information (CQI) of each SS, that is, channel states, can be exactly measured. Further, a Base Station (BS) minimizes inter-beam interference by means of correlation of the preamble sequence, and generates an exact beam based on channel states estimated according to each SS, thereby preventing signals targeting each SS from acting as an interference to other SSs.
FIG. 1 is a diagram schematically illustrating the general structure of an IEEE 802.16e communication system using an SDMA scheme.
Referring to FIG. 1, a BS 101 allocates the same time and frequency resources so that different SSs can simultaneously use the time and frequency resources in both a first space channel transmitted through a first beam 102 and a second space channel transmitted through a second beam 103. In order to allocate the same time and frequency resources to the different SSs, the BS 101 generates a plurality of spatially separated beams.
In order to generate a beam for a downlink, exact uplink channel states are required. Accordingly, the general IEEE 802.16e communication system transmits an AAS preamble sequence to a downlink and an uplink in order to support an AAS, thereby having knowledge of the exact downlink and uplink channel states.
FIG. 2 is a diagram schematically illustrating the general frame structure of an IEEE 802.16e communication system.
Referring to FIG. 2, the frame is classified into a downlink frame 200 and an uplink frame 250. The downlink frame 200 includes a downlink preamble field 211, a Frame Control Header (FCH) field 213, a downlink MAP (DL-MAP) field 215, an uplink MAP (UL-MAP) field 217, a plurality of AAS preamble fields 219, 221, 223 and 227, and a plurality of downlink burst fields, i.e., a first downlink burst field 225, a second downlink burst field 229, a third downlink burst field 231 and a fourth downlink burst field 233.
The downlink preamble field 211 is a field to which synchronization signals (i.e., a downlink preamble sequence) are transmitted in order to acquire synchronization (i.e., transmission/reception interval) between a BS and an SS. The FCH field 213 is a field through which basic information for a sub-channel, such as a ranging, a modulation scheme, etc., is transmitted. The DL-MAP field 215 is a field through which a DL-MAP message is transmitted, and the UL-MAP field 217 is a field through which a UL-MAP message is transmitted. Herein, because Information Elements (IEs) included in the DL-MAP message and the UL-MAP message have no direct connection to the present invention, a detailed description will be omitted. The AAS preamble fields 219, 221, 223 and 227 are fields through which downlink AAS preamble sequences for AAA support are transmitted, and the downlink burst fields 225, 229, 231 and 233 are fields through which downlink data targeting SSs are transmitted.
The uplink frame 250 includes a plurality of AAS preamble fields 251, 253, 255 and 259, and a plurality of uplink burst fields, i.e., a first uplink burst field 257, a second uplink burst field 261, a third uplink burst field 263 and a fourth uplink burst field 265. The AAS preamble fields 251, 253, 255 and 259 are fields through which uplink AAS preamble sequences for AAA support are transmitted, and the uplink burst fields 257, 261, 263 and 265 are fields through which uplink data targeting a BS are transmitted from the SSs.
The BS estimates uplink channel states through the uplink AAS preamble sequences, and generates a downlink beam according to the estimated uplink channel states. In the IEEE 802.16e communication system, the AAS preamble sequences have been defined as different sequences in each space channel, i.e., each beam. However, because the IEEE 802.16e communication system has not proposed a scheme for controlling transmit power used for transmitting the AAS preamble sequence, it is impossible to understand a transmit power relation between the AAS preamble sequence and a data burst. Therefore, normal uplink data decoding is impossible. Accordingly, for the IEEE 802.16e communication system, it is necessary to provide a scheme for controlling transmit power used for transmitting the AAS preamble sequence.